1. Field of the Invention
The present invention relates generally to an analog quadrature modulation (AQM) apparatus and method in a base station transmitter of a wireless mobile communication system. In particular, the present invention relates to an AQM error compensation apparatus and method for compensating for an error in an AQM output signal.
2. Description of the Related Art
For the effective use of frequency resources increasingly exhausted in a wireless mobile communication environment, linear modulation such as M-ary Phase Shift Keying (M-PSK) and M-ary Quadrature Amplitude Modulation (M-QAM) is needed to increase frequency efficiency (or spectral efficiency).
Generally, a high-power amplifier serves as an important part of amplifying a radio frequency (RF) signal and transmitting the amplified RF signal to a base station over the air, and has a large influence on the non-linearity of the entire system.
To improve the non-linearity characteristic of the power amplifier, there have been proposed a feed forward technique, an envelop feedback technique, and a predistortion technique. Among such linearization techniques, the predistortion technique is the most inexpensive due to its performance and operation at a wider bandwidth.
The predistortion technique pre-distorts an input signal in opposition to the non-linearity characteristic of a power amplifier, and provides the predistorted signal to the high-power amplifier, thereby improving linearity. The predistortion technique, which can be implemented at a baseband, can improve the size and efficiency of the entire system.
In implementing such a linearization apparatus, an analog quadrature modulation (AQM) unit up-converts I and Q signals acquired through a predistortion unit, a digital-to-analog converter (DAC) and a low-pass filter (LPF) into a radio frequency (RF) signal or an intermediate frequency (IF) signal. However, an output signal of the AQM unit has a gain/phase imbalance and a Direct Current (DC)-offset error. For this reason, digital predistortion cannot show a linearization function of the high-power amplifier, so that a transmission stage cannot satisfy a spectral emission standard and a reception stage causes an increase in the bit error rate (BER), thereby resulting in performance degradation of the entire system.
Therefore, various techniques for effectively compensating for an AQM error in implementing the linearization apparatus have been proposed.
In particular, James K. Cavers has proposed an AQM error compensation technique that uses a transmission signal of an intact transmission stage instead of using a particular test signal. A block diagram of a linearization apparatus including the AQM error compensation apparatus proposed by James K. Cavers will be described with reference to FIG. 1.
Referring to FIG. 1, the linearization apparatus includes a digital predistortion (DPD) unit 110, an error compensator 120, first and second digital-to-analog converters (DACs) 130 and 135, first and second low-pass filters (LPFs) 140 and 145, an analog quadrature modulation (AQM) unit 150, a band-pass filter (BPF) 160, a high-power amplifier (HPA) 170, an envelope detector 180, an analog-to-digital converter (ADC) 185, and an error estimator 190. The digital predistortion unit 110 predistorts an input signal such that it has a characteristic opposite to a non-linear distortion characteristic of a digital input signal. The error compensator 120 receives digital I/Q signals output from the digital predistortion unit 110 and compensates for AQM error signals in the received digital I/Q signals. The first and second digital-to-analog converters 130 and 135 convert the digital I/Q signals from the error compensator 120 into analog I/Q signals, respectively. The first and second low-pass filters 140 and 145 low-pass-filter the analog I/Q signals output from the first and second digital-to-analog converters 130 and 135, respectively. The analog quadrature modulation unit 150 modulates the analog I/Q signals output from the first and second low-pass filters 140 and 145 with a carrier frequency. The band-pass filter 160 band-pass-filters the output signals of the analog quadrature modulation unit 150. The high-power amplifier 170 amplifies a signal output from the band-pass filter 160. The envelope detector 180 is located in a feedback path of the signal output from the band-pass filter 160. The analog-to-digital converter 185 digital-converts the analog signal from the analog quadrature modulation unit 150, output from the envelope detector 180, and outputs sample values. The error estimator 190 extracts a gain/phase imbalance and a DC-offset error of the samples output from the analog-to-digital converter 185 using reference I/Q signals received from the predistortion unit 110, and applies the extracted gain/phase imbalance and DC-offset error to the error compensator 120.
The structure of FIG. 1 simultaneously predicts the gain/phase imbalance and DC offset, and compensates the digital pre-distorted output signal for three types of error components.
In the case of the technique proposed by James K. Cavers, a digital stage simultaneously predicts three types of error components using a transmission signal and even an envelope detector used for a feedback stage is included in system modeling for a prediction algorithm, thereby increasing algorithm reliability.
However, the structure of FIG. 1 is commercially unfeasible due to its complicated adaptive calculation process, taking into consideration that it is implemented with a fixed point in an actual digital signal processor (DSP). In addition, when a particular test signal is used for error compensation, in order to continuously manage a base station transmitter with an initially compensated value or in order for an algorithm to adapt itself to a change in external conditions, it is necessary to interrupt transmission for a predetermined time. For this, a user should perform correction on a trial-and-error basis, or should turn on a computer and perform correction based on predetermined values, preventing preferable management of a base station.
In addition to the error compensation technique illustrated in FIG. 1, there are various other techniques according to a technique of using a signal fed back from the high-power amplifier 170 for a prediction algorithm, compensation order of defects, use of a particular test signal rather than a transmission signal, and compensation at an analog stage.
However, even when an AQM error is compensated for using a feedback signal from the high-power amplifier, it is necessary to include a correct power amplifier model in system modeling for an algorithm, causing an increase in hardware complexity.
In addition, because the algorithm needs exact parameters such as a gain and a DC offset of the envelope detector, it is difficult to implement a substantial adaptive algorithm. Also, performing compensation through independent processes on a one-by-one basis and performing compensation at an analog stage in order to compensate for three types of errors are not suitable for a stable and efficient system integration for the algorithm.